In a known type of nuclear power reactor, for example, a boiling-water reactor, nuclear fuel is provided in the shape of elongated rods. The nuclear fuel is typically in the form of uranium and/or plutonium oxide pellets enclosed in zircaloy tubes. A number of such fuel rods are grouped together in an open-ended tubular flow channel. The flow channel with the fuel rods positioned therein is referred to as a "fuel assembly" or "bundle." A plurality of fuel assemblies are removably positioned in the reactor core in a matrix. The reactor core formed in this manner is capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
The fuel rods in a fuel assembly are supported between upper and lower tie plates. The fluid coolant flows past the fuel rods in the inter-rod spaces. To provide proper coolant flow and preserve integrity of the fuel rods, it is important to maintain the rods in a preferred spaced relationship and to restrain them from bowing and vibrating during reactor operation.
To maintain uniform spacing, a plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose. Typically, a spacer includes a plurality of generally cylindrical ferrule elements. An example of a spacer usable in a fuel assembly is that shown in U.S. Pat. No. 4,508,679, issued Apr. 2, 1989, to Matzner, et al. As shown in Matzner, et al., one method of positioning a fuel rod within the ferrule elements of a spacer is to provide a spring member for biasing the fuel rod against rigid stops in the ferrules. The spring depicted in U.S. Pat. No. 4,508,679 is in the form of a continuous loop of generally elliptical shape. The springs are positioned in the area between rods.
Previous fuel assemblies had been constructed with rod-to-rod spacings (i.e., distances between outer circumferences of adjacent rods) of about 0.125 inches to about 0.160 inches (about 3 mm to about 4 mm). Modern fuel bundles are being designed to have a reduced rod-to-rod spacing, such as less than about 0.11 inches (about 2.8 mm), preferably about 0.1 inches (about 2.5 mm). Such spacings are encountered in modern fuel rod arrays having matrix densities of 9 by 9 or higher.
The length and width of the springs is also limited by the spacer design. The spring length must be less than the spacer height in order that the spring can be captured in the spacer. If the spacer height is increased, the pressure drop through the spacer is increased. If the spring width is increased, the spring will block more flow area and cause an increased pressure drop.
A nominal spring force of about 2.5 pounds is required. During assembly and shipping, deflections greater than the nominal deflection can be imposed. In addition, dimensional variations from the nominal values can impose increased deflections. The spring must be able to absorb these additional deflections without suffering permanent deflection.
In summary, the spring should provide a given nominal force and be able to absorb deflection beyond the nominal value, while fitting into a small space. The spring disclosed here provides the required nominal force and has a greater deflection before the onset of permanent deflection than the loop spring, under the design constraints described above.